Ionizing radiation has been used in the oilfield for well over half a century to enable measurements not possible with other formation probing techniques (such as electrical resistivity, acoustic, electro-magnetic, NMR, etc.). The broad types or kinds of ionizing radiation include charged particles, X-rays, gamma-rays, and neutrons.
Historically, the first method employed to produce ionizing radiation for oilfield applications involved radioactive sources. These radioactive sources (e.g. 137Cs, 241Am, 252Cf, 238Pu, etc.), were chosen, based on the application, for their emitted radiation type and in quantities sufficient for the desired intensity. Few naturally occurring radioactive sources emit neutrons during their decay, generally as a consequence of spontaneous fission. One example of a radioactive material with a significant probability for spontaneous fission is 252Cf. While 252Cf is used in some applications, its short half-life of only 2.6 y is generally considered a drawback. Many neutron sources rely on the generation of neutrons through a nuclear reaction, typically the 9Be(alpha, n)12C reaction. Such sources consist of an alpha emitter, such as 241Am or 238Pu, mixed with Be-powder. The small nuclear reaction probability necessitates the use of a large amount of radioactive alpha emitter to generate a sufficient neutron output.
The small size and simplicity of the source housing, the reliability of continuous radioactive decay, and the typically low purchase cost make such radiation sources very attractive. The shortcomings, however, include limited radiation output, (particularly for neutron sources), issues of radiation safety because the source remains ON at all times, as well as the increase in the cost of the material and its disposal, the accountability (tracking/inventory requirements) and, for some like 252Cf, a short useful life. The absence of the ability to fast pulse ON/OFF a chemical source also limits the measurements that can be made.
To overcome these disadvantages of radioactive sources, electronic radiation generators were developed over the last several decades. Electronic radiation generators function by accelerating and colliding charged particles inside of sealed tubes to induce radiation-producing reactions. The two approaches to accelerating charged particles are to use linear accelerators (e.g., Linac) or circular accelerators (e.g., cyclotron). The linear accelerators employed in the oilfield, and of relevance to this particular invention disclosure, are electrostatic-based. Linear accelerators operate by creating a sufficiently large voltage difference between a source of charged particles and a target. Once the source is turned ON, charged particles entering the region of high voltage (HV) are accelerated to the predetermined desired energy and made to collide with a suitable target material to produce radiation. If either the ion source or the voltage are turned OFF, the radiation stops. The accelerated charged particles can be electrons, protons and heavier ions, which will generate ionizing radiation when they impinge on the target.
The intensity and energy of the radiation produced depends on the high voltage used to accelerate the charged particles. Typically, the higher the “high voltage” is, the higher the energy of the charged particles. High voltage power supplies capable of producing from tens to hundreds of kilovolts are desirable. A common power supply design employed is based on the Cockcroft-Walton voltage multiplier, also known as the Greinacher multiplier. Such a voltage multiplier includes an array of stages in series, shaped like a ladder, with each stage 100 comprising a pair of capacitors 102 and a pair of diodes 104 (see FIG. 1).
During each ON cycle, the A/C current supplied by the step up transformer 106 charges each respective capacitor 102, according to the rectification provided by the diodes 104. The maximum admissible voltage for each stage 100 depends on the capacitor and diode component ratings as well as on the AC voltage provided by the secondary of the step up transformer 106. To achieve a given high voltage, a ladder is constructed such that, to first order (i.e., neglecting losses, etc.), enough stages are used to sum up to the predetermined desired voltage. The high voltage output 108 of the series of stages, or ladder, can then be fed to a voltage divider (bleed resistors, discussed in further detail in related application Ser. No. 61/261,398, non-provisional application filed concurrently herewith, to obtain a measurement of the high voltage fed out to neutron generator tube.
One electrical constraint for the design of high voltage power supplies is that electrical hold off between high voltage and ground must be sufficient over the whole length of the power supply. Given the limited diameter available in downhole tools, the stage component size must be small enough to accommodate the necessary high voltage insulation over a very limited radial distance. Typically, the size of high voltage components increases with their voltage rating. A tradeoff must be made between the stage (and component) size and the necessary high voltage insulation based upon the voltage difference to neighboring ground potential. For borehole-size radiation generators, high voltage capacitor ratings are typically on the order of around a few kilovolts (e.g., five kilovolts). Typically, multi-layer capacitors are used to achieve the required capacitance, and are generally rectangular or box-shaped with radial leads. Thus, typically to produce from tens to hundreds of kilovolts requires a power supply with several dozens of stages. When the stages are placed end to end, this adds up to a significant overall length for the power supply.
Currently, the most frequently employed electronic radiation generator in the oilfield is the pulsed neutron generator (PNG), as shown in FIG. 2. Aside from the supporting low voltage electronics (current and pulsing supplies), a PNG comprises a high voltage power supply 202 (comprising comprises the step up transformer and the voltage multiplier), a sealed neutron generator tube 204 and a high voltage insulation system 206. The sealed neutron generator tube 204 includes an ion source 208 which is at or near ground potential, and separated from a target 210 by an HV acceleration column. The configuration of a PNG is generally cylindrical, as dictated by the borehole geometry, with the target 210 of the neutron generator tube 204 in contact with the high voltage end of the high voltage power supply 202. The necessary high voltage insulation (solid, liquid and/or gaseous or any combination thereof) surrounds the neutron generator tube 204 and high voltage power supply 202, with the entirety being contained in a hermetically-sealed pressure housing 212 (not to be confused with an overall tool pressure housing of a tool in which the present generator is utilized). Preferably, the pressure housing is cylindrical in shape. The conventional arrangement of these components is in a rectilinear configuration, where the high voltage power supply 202 is connected in line (on one axis or close to on one axis) to the high voltage end of the neutron generator tube 204 (see FIG. 2), frequently with a protective surge resistor 214 therebetween. The typical length of such a generator is often in the range from about 36 to 48 inches, or more.
The considerable length of a typical PNG configured as in FIG. 2 can impose significant limitations on the tool design and consequently the range of measurements possible to obtain. Because of the length of the high voltage power supply in the typical PNG, only the neutron generator tube end affords close proximity to the radiation source (i.e., the target). For most practical purposes, this limits detector placement to a single axial direction relative to the target. If a tool is intended to perform multiple different measurements with the radiation provided by the electronic source, it is likely that not all the required detectors can be placed at the optimal axial and/or radial position. A short radiation source would allow detector placements axially on both sides of the radiation source, and thus a better independent optimization of multiple measurements through optimal placement of multiple detectors.